The Founders, the founding documents, the Supreme Court, and common sense for starters. Do you really seriously believe that they went through all that trouble in order to create a transitory or perishable union? From Texas v. White:
By [the Articles of Confederation], the Union was solemnly declared to "be perpetual." And when these Articles were found to be inadequate to the exigencies of the country, the Constitution was ordained "to form a more perfect Union." It is difficult to convey the idea of indissoluble unity more clearly than by these words. What can be indissoluble if a perpetual Union, made more perfect, is not?It should be self-evident.
“It should be self-evident.”
That is precisely the problem. The words of founding documents and the SCOTUS decision are so self-evident, giving them any acknowledgement shatters the illusions of his fantasy.